This invention relates to a method of making asymmetric Langmuir-Blodgett films by depositing monomolecular layers on a substrate. More specifically, this invention comprises a method for depositing two or more different monomolecular layers onto a substrate.
Monomolecular layers of organic compounds find a large number of applications, especially in the field of electronics. For example, monomolecular layers are used in forming metal-insulator-metal structures in which the thickness of the dielectric insulator layer must be controlled with a particularly high degree of accuracy.
One known method for producing and depositing monomolecular layers of amphiphillic molecules is the well known Langmuir-Blodgett method, described in the Journal of The American Chemical Society, Volume 57 (1935) pages 1007-1010.
The Langmuir-Blodgett method consists of forming a monomolecular layer at the surface of a tank filled with a liquid subphase such as water. In the Langmuir-Blodgett method a solution of amphiphillic molecules dissolved in a solvent which is not miscible with the subphase liquid in the tank is spread onto the liquid surface. Amphiphillic molecules are those having a hydrophobic first end and a hydrophiphillic second end lined up side by side in a particular direction. When the solvent evaporates a loosely packed monomolecular layer is formed on the surface of the subphase. A transition of the monomolecular layer thus formed from a state of gas or liquid to a solid state is then achieved by reduction of the surface area of the layer by compressing it to a predetermined surface pressure. The resulting monomolecular layer is deposited onto the surface of a substrate by passing the substrate through the compressed layer while maintaining the layer at a predetermined surface pressure during the period of deposition.
A standard method for compressing the monomolecular layer consists of using a leak tight moving barrier which moves over the liquid surface of the tank. By displacing the barrier, the area of the monomolecular layer which had previously been formed at the liquid surface of the tank is reduced to bring the layer to the desired surface pressure.
In a conventional Langmuir-Blodgett system, the substrate is passed through the monomolecular layer on the surface of the subphase to coat the substrate with a single layer. On the return trip through the subphase, a second layer is applied, and this second layer is undesirable for many applications. In the method provided in accordance with this invention, the substrate is passed through each of a plurality of monomolecular layers in only one direction, so that only a single coating of a particular layer is applied.
Other Langmuir-Blodgett systems are described in a number of U.S. Patents. For example, Albrecht et al U.S. Pat No. 4,722,856 discloses a system for maintaining the pressure of a monomolecular layer on the surface of a subphase substantially constant. Barraud et al U.S. Pat. No. 4,599,969 discloses a tank with a single trough having a movable barrier for controlling surface pressure of the monomolecular layer. The mechanical arrangement of Barraud's barrier would be suitable for use in each of the troughs used in accordance with this invention. Ono, U.S. Pat. No. 4,779,562 shows a system in which the substrate is held fixed while the subphase is raised, thereby eliminating the need for lowering and raising the substrate.
None of the known prior art uses a two trough system in which the substrate is first coated with a single monomolecular layer by first immersing the substrate through a layer on the surface of a common subphase in one of the troughs, then moving the substrate laterally below the surface to the second trough, where it is coated with a second different layer by withdrawing the substrate from the subphase and through the second layer.